Bones provide rigid support for the body, maintain mineral homeostasis, and serve as the primary site for hematopoiesis. Bone homeostasis is maintained through the balanced and coupled actions of osteoblasts and osteoclasts. Bone-resorbing osteoclasts are derived from myeloid-lineage precursor cells, which also yield immunocytes such as macrophages and dendritic cells. This application stems in part from our attempt to understand how osteoclasts differ functionally and developmentally from immunocytes. In our preliminary data, we describe a novel subunit isoform of v-ATPase of previously unknown function, Atp6v0d2, expression of which is highly enriched in osteoclasts. We show that Atp6v0d2 knockout mice exhibit mild osteopetrosis due to decreased bone resorption. To our surprise, however, Atp6v0d2 does not appear to be involved in the v-ATPase proton pump, which is required for osteoclast-mediated bone resorption. Rather, Atp6v0d2 is critical for the optimal generation of multinucleated osteoclasts. In addition, Atp6v0d2 knockout mice exhibit increased bone formation, even though Atp6v0d2 expression was not detected in osteoblasts. Moreover, when tested directly ex vivo, Atp6v0d2-deficient osteoblasts do not show any intrinsic defects in their differentiation or mineralization of extracellular matrix. These observations strongly suggest that Atp6v0d2 knockout mice show enhanced bone formation as a result of unknown osteoblast-extrinsic mechanisms, possibly via Atp6v0d2-deficient osteoclast lineage cells. Finally, by examining Atp6v0d2 expression, we show that NFATc1 may regulate cell-cell fusion between preosteoclasts, in part, by regulating the induction of Atp6v0d2 expression, suggesting that signaling cascades that regulate the differentiation of preosteoclasts from myeloid precursors may be active during multiple stages of osteoclast development. Therefore, we propose to extend these studies of the regulation of bone homeostasis and osteoclast maturation by pursuing the following specific aims: (1) determining the extent to which Atp6v0d2 deletion affects bone metabolism in vivo, (2) determining whether osteoclast lineage cells per se can increase bone formation in vivo, and (3) extending our understanding of osteoclast maturation by studying signaling cascades that regulate Atp6v0d2 expression and the multinucleation of preosteoclasts. The knowledge gained from these studies will provide insights into how different molecules cooperate to induce osteoclast differentiation, and how osteoclasts and osteoblasts communicate to cross-regulate their functions, which may lead to novel or improved therapeutic strategies for the treatment and prevention of osteoporosis and other bone diseases.